|An alkene is simply a C=C, that is a double bond between two carbon
atoms. The position of the double bond will often determine what is the
parent chain. If a double bond exists then the parent chain must include
it. The double bond also uses up 2 bonds that normally hold hydrogen so the
general formula for alkenes is CnH2n. The double bond
also locks the molecule in a certain position so that attached groups can't
rotate around the double bond. This means some special naming conventions.
Easy stuff first:
Butene CH2=CH-CH2-CH3 or CH3-CH=CH-CH3
|There are two structural formulas for butene. This is because the
double bond can come between the 1st and 2nd C or
between the 2nd and 3rd carbon. We get around this
by naming the position of the "ene". You'll also have noticed that the names
have changed from "ane" to "ene" indicating the presence of a double bond.
|These two are isomers. They have the same molecular formula, C4H8, but different structural formulas. We also have a new problem.|
|The cis means (same side) and the CH3-
methyl groups are both above the C=C bond and on the same side (top). Trans
means transverse and the CH3- methyl groups
are in a trans position to each other across the C=C bond.
|Some more examples:
|The longest chain of carbons is 7. Therefore this is a "hept" molecule.
There is also a C=C double bond therefore it is a heptene. Counting from
both ends the double bond starts at either 3 or 4. Choose the lowest number.
The molecule is now hept-3-ene. The longest chain is also split around the
C=C bond in a transverse fashion therefore it is also trans-hept-3-ene.
|This accounts for everything about the parent chain. Since we choose
the C=C number to be 3 we have now locked in place a numbering system for
this molecule. That means there is a methyl group at carbon 4. The name
of the molecule is therefore: 4-methyl-trans-hept-3-ene.
|Look for the longest continuous carbon chain. There are 9 carbons therefore it is a "nona" and there is a C=C at carbon 4 therefore it is non-4-ene. The sides of the chain of the parent are both on the same side. In this case, on top of the double bond therefore it is cis-non-4-ene. There is a methyl group at carbon 4 and an ethyl group at carbon 5. Therefore the full name is 5-ethyl-4-methyl-cis-non-4-ene.|